Production methods of resin films mainly include a melt-extrusion molding method, a solution flow casting method, a calendering method and the like. The solution flow casting method among these is such a method that a solution having a resin dissolved in a solvent (a solvent solution of the resin) is flow-cast on a support, such as a drum and a belt, and the solvent in the solvent solution is evaporated by drying to provide a resin film. In the solution flow casting method, the film does not receive a physical force to provide such characteristics that the polymer is difficult to suffer orientation to prevent directionality from occurring in strength and optical characteristics, and thus the method is suitable for producing a resin for optical purposes. However, it is necessary to expose the solution to a high temperature exceeding 240° C. upon evaporating the solvent by drying in some kinds of solvents, and the resulting resin film suffers decrease in total light transmittance and increase in haze depending on the kind of the resin, which become a significant problem in optical purposes.
Polymethyl methacrylate, which has been used as optical plastics, has low birefringence and colorless transparency, but is insufficient in heat resistance and thus cannot be applied to an optical purpose where high heat resistance is required. Polycarbonate has a relatively high glass transition temperature, but cannot be applied thereto since it does not necessarily satisfy heat resistance that is required in an optical purpose and has large birefringence.
It is known that a polyimide has high heat resistance. An ordinary polyimide is obtained from an aromatic tetracarboxylic anhydride and an aromatic diamine and is excellent in heat resistance, chemical resistance, mechanical property and electric characteristics owing to the rigidity of the molecule, the resonance stability and the strong chemical bond, and therefore it is widely used in such fields as molded articles, composite materials, electric and electronic members and the like. In addition, a polyimide capable of being melted under heat using 1,2,4,5-cyclohexanetetracarboxylic anhydride and a reactive derivative thereof (see Patent Document 1) and a colorless transparent polyimide obtained by decoloration (see Patent Documents 2 and 3) have been also developed.    [Patent Document 1] U.S. Pat. No. 3,639,343    [Patent Document 2] JP-A-8-143666    [Patent Document 3] JP-A-8-225645